Male Reproductive System Lecture Notes

Overview of the Male Reproductive System

The male and female reproductive systems are composed of a complex series of glands and tubes designed to produce and nurture sex cells, as well as transport them to the specific site of fertilization. In the male, the reproductive organs are specifically adapted to transport sperm cells to the female egg cells. These organs are classified into two categories: primary sex organs and accessory sex organs. The primary sex organs, also known as the gonads, are responsible for the production of sperm and the secretion of hormones. The accessory sex organs serve a supportive function in the overarching process of reproduction. Physical structures identified in the male reproductive anatomy include the urinary bladder, ureter, large intestine, seminal vesicle, ejaculatory duct, prostate gland, bulbourethral gland, anus, epididymis, testis, scrotum, prepuce, glans penis, penis, corpus spongiosum, corpus cavernosum, urethra, vas deferens, and the symphysis pubis.

Anatomy and Physiology of the Testes

The testes are ovoid structures that are suspended within the scrotum by a structure known as the spermatic cord. Internally, each testis is organized into approximately $250$ lobules, which are separated from one another by layers of connective tissue. Within each individual lobule, there are between $1$ and $4$ highly coiled seminiferous tubules. These tubules are lined with a specialized stratified epithelium that is responsible for giving rise to sperm cells. Located in the spaces between the seminiferous tubules are the interstitial cells, also referred to as cells of Leydig, which produce male sex hormones known as androgens.

Sperm Cell Structure and Morphology

A mature sperm cell consists of three primary regions: the head, the midpiece, and the flagellum. The head contains the haploid nucleus, which carries the genetic material. At the very tip of the head is the acrosome, described as a bag of digestive enzymes. These enzymes are essential for eroding the tissues that surround the female egg cell to allow for penetration. The midpiece of the sperm cell is packed with mitochondria, which are responsible for generating the adenosine triphosphate ($\text{ATP}$) necessary for movement. The flagellum, or tail, provides the motility required for the sperm to swim.

The Process of Spermatogenesis

Spermatogenesis is the process by which sperm cells are produced, a cycle that typically lasts between $65$ and $75\text{ days}$. The process begins with spermatogonia, which are undifferentiated spermatogenic stem cells present in the male embryo. Each spermatogonium contains a full complement of $46$ chromosomes. During the course of spermatogenesis, these spermatogonia enlarge and transform into primary spermatocytes, which also contain $46$ chromosomes. These primary spermatocytes then undergo division via meiosis to form haploid secondary spermatocytes, which possess $23$ chromosomes. The secondary spermatocytes divide once more to form spermatids. Each spermatid eventually matures into a fully formed sperm cell ($23$ chromosomes). While the development of sperm begins during the fetal stage, these cells do not reach full maturity until the onset of puberty. This developmental sequence moves from the wall of the seminiferous tubule toward the lumen.

Male Internal Accessory Organs

The internal accessory organs facilitate the maturation and transport of sperm. The epididymis serves as the specific site where sperm maturation occurs. Leading away from the epididymis is the vas deferens, a muscular tube measuring approximately $45\,cm$ in length. The vas deferens travels from the epididymis up into the body cavity toward the ejaculatory duct, where it eventually empties its contents into the urethra.

Secretory Glands and Fluid Composition

Several glands contribute secretions to the reproductive fluid. The seminal vesicles are attached to the vas deferens near the base of the urinary bladder. They secrete an alkaline fluid that contains fructose to provide nourishment for the sperm, prostaglandins to induce muscular contractions in the female tract to help propel the sperm and improve motility and viability, and clotting proteins to help semen coagulate following ejaculation. This alkaline secretion also helps to neutralize the acidic environments found in both the male urethra and the female reproductive tract.

The prostate gland is a chestnut-shaped organ located at the base of the urinary bladder, surrounding the urethra. It secretes a thin, milky, and slightly acidic fluid with a $\text{pH}$ of approximately $6.5$. This fluid contains citric acid for $\text{ATP}$ production via the Krebs cycle, as well as various proteolytic enzymes including pepsinogen, lysozyme, amylase, and prostate-specific antigen ($\text{PSA}$). These enzymes work to break down the clotting proteins secreted by the seminal vesicles. $\text{PSA}$ is known to leak into the bloodstream; the larger and more active the prostate is, the higher the concentration of $\text{PSA}$ in the blood, though this is not considered a specific test for prostate cancer. The prostate also secretes acid phosphatase, though its function is currently unknown. Overall, prostatic secretions serve to enhance the mobility and viability of sperm cells. Finally, the bulbourethral glands are small structures located inferior to the prostate that secrete mucus to provide lubrication to the tip of the penis during periods of sexual arousal.

Characteristics and Dynamics of Semen

Semen is a composite fluid made of sperm cells and the combined secretions of the seminal vesicles, prostate gland, and bulbourethral glands. A single ejaculation typically results in a volume of $2.5$ to $5\,mL$ of semen, containing between $50$ and $150\text{ million sperm per mL}$. The $\text{pH}$ of semen ranges from $7.2$ to $7.7$, a result of the high volume of alkaline fluid from the seminal vesicles. Semen also contains seminalplasmin, an antibiotic that can destroy certain bacteria.

Before a sperm cell is capable of fertilizing an egg, it must undergo a process known as capacitation within the female reproductive tract. This involves a series of functional changes that cause the sperm's tail to beat vigorously and prepare the plasma membrane to fuse with the oocyte membrane. Specifically, secretions within the female tract remove cholesterol, glycoproteins, and proteins from the plasma membrane surrounding the acrosome. Additionally, semen exhibits specific physical changes post-ejaculation: it coagulates within $5\text{ minutes}$ due to the clotting proteins from the seminal vesicles, and then reliquefies within $10$ to $20\text{ minutes}$ as $\text{PSA}$ and other proteolytic enzymes break those proteins down.

Hormonal Control of Male Reproduction

At the onset of puberty, male reproductive functions are regulated by a hormonal cascade. The hypothalamus releases gonadotropin-releasing hormone ($\text{GnRH}$), which triggers the anterior pituitary gland to produce two gonadotropins: luteinizing hormone ($\text{LH}$) and follicle-stimulating hormone ($\text{FSH}$). LH promotes the development of the interstitial cells in the testes, which in turn secrete testosterone. FSH stimulates the supporting cells within the seminiferous tubules to begin spermatogenesis. Testosterone itself also independently stimulates spermatogenesis.

Testosterone is the most abundant male sex hormone. It is secreted during fetal development until birth, stops, and then is secreted continuously starting at puberty. Testosterone stimulates the development of male reproductive organs, causes the testes to descend into the scrotum, and is responsible for male secondary sexual characteristics, including a deepening of the voice, increased body hair, thickening of the skin, and the broadening of the shoulders.

Physiological Mechanisms of Sexual Response

The male sexual response is triggered by sexual stimulation, which may be visual, tactile, auditory, olfactory, or imagined. During an erection, parasympathetic impulses from the sacral region of the spinal cord cause the release of nitric oxide. This leads to the vasodilation of the arteries that supply the penis, allowing large volumes of blood to enter the sinuses of the erectile tissue. This expansion compresses the veins, slowing the outflow of blood. Ejaculation is a somatic motor action where potentials are sent to the skeletal muscles of the diaphragm and the base of the penis, resulting in rhythmic contractions that force semen through the urethra. This process is accompanied by an overall increase in muscle tension throughout the body.